(1) Field of the Invention
The present invention relates to a liquid measuring device for dispensing measured volumes of liquid from a source of liquid under pressure. More specifically, the present invention relates to a liquid measuring device of the type having a cylinder and piston, with chambers on either side of the piston, each defining the volume to be dispensed.
(2) Description of the Prior Art
In many applications, it is desirable to measure an accurate and precise volume of liquid to be dispensed. In the packaging industry in particular, it is desirable to provide a measuring device that measures liquid accurately and precisely. In the packaging field, standards often exist for filling containers with liquid to insure that they contain at least the amount specified. For example, if an apparatus fills a container to a set volume, which varies positively or negatively from the volume set on the dispensing apparatus, many governmental bodies require that the apparatus be set slightly higher so that a statistically high number of containers have a volume that is at or exceeds the desired volume. In the packaging industry, it is common for a dispensing device to be rigorously tested to determine its precision, and with a statistically relevant sample, determine the standard deviation for the volumes being dispensed. Once the standard deviation is determined, it is typical for a governmental body to require that the apparatus be set at the volume desired plus a multiple of the standard deviation of a statistically relevant sample. It is not uncommon for dispensing devices to be set at volume levels that are 0.25% or greater of the volume being dispensed. While this volume may seem small, when hundreds of thousands to millions of containers are filled, the quantity of liquid dispensed in excess of the labeled amount becomes a costly factor.
U.S. Pat. No. 384,766 to Kendall and U.S. Pat. No. 1,144,583 both disclose a liquid measuring device having a double acting piston that is operated by a four-way valve. Liquid under pressure is supplied through a four-way valve to both chambers of the cylinder. Liquid is admitted one chamber and moves the piston to dispense a volume of liquid from the other chamber. U.S. Pat. No. 4,036,409 and U.S. Pat. No. 1,995,342 also disclose liquid measuring devices of a similar type.
In prior art measuring devices, the piston is typically made of solid material, such as, for example, stainless steel and has a specific gravity that is substantially greater than the fluid being dispensed. For example, the Langlais patent discloses a fluid metering device that has a piston which moves vertically within a cylinder. On the upstroke of the piston, the fluid supplied to the lower chamber tends to leak through a clearance between the piston and the cylinder thus adding a small incremental volume to the volume being dispensed from the upper chamber. On the downward stroke of the piston, movement of the piston is aided by the weight of the piston itself and the volume dispensed from the lower chamber is decreased by a small incremental volume because of leakage from the lower chamber into the upper chamber.
In prior art free piston devices, the piston may be sealed to the cylinder by flexible seals, piston rings, or other sealing means. If this is done by a highly accurate measure can be achieved. The problem encountered with seals however include the following:
(1) The seals tend to wear causing leakage, maintenance costs and contamination of product.
(2) The seals require much higher fluid pressure to operate, particularly when handling liquids with low lubricating qualities.
(3) The seals may result in trapping of air within measuring chambers from product which will adversely affect accuracy.
It is an object of the present invention to provide a liquid dispensing device which provides extremely accurate and precise dispensing of liquid. It is another object of the present invention to provide a measuring device which does not require seals between the piston and the wall of the cylinder along which the piston moves, thereby avoiding the necessity of changing the seals regularly. It is a further object of the invention to provide a "free" piston type device, i.e. that is, one which operates on the pressure of the liquid being the dispensed.